One of the drawbacks of owning an HK USP Compact .45 ACP  is that the largest standard magazine for it holds just 10 rounds, and to date, none of the magazine makers produce anything larger. This puts the HK at a disadvantage to guns like the Glock Model 30. (Which can of course accept the longer Glock M21 magazines that hold 13 rounds.) Sadly, since they use substantially different frame form factors, the HK USP Compact .45s cannot accept the fatter HK full-size USP .45 ACP magazines, which are made up to 12 round capacity.
But now, after a bit of gentle prodding from your editor, Taylor Freelance  in Washington now makes a 4-round “Rawles Special” magazine extension for the 10-round HK USP Compact .45 ACP magazine. These are precision machined from solid aluminum and provide 14+1 capacity. The prototypes that Taylor custom made for me have functioned flawlessly. Installing them took just a couple of minutes, per magazine. No modification to the magazine body is required. Since the HK 45C  uses the same magazine, these extensions will also work with the HK 45C pistol. Also note that they will not work with the eight-round magazine, since it has a different method of floorplate attachment.
At $55 each (with a long spring provided for each), these extensions could be considered “spendy”, especially when you consider that the HK factory 10 round magazines that these fit on cost around $45 each, just to start. However, when your life is on the line and you need couple of extended spare magazines as backups for your USP Compact, then these could put you in that in omnia paratus comfort zone. They could also be very useful for competitive shooters. (Among other things, their bit of extra weight makes fast, positive magazine ejection a certainty.)
See the Taylor Freelance  web site for their magazine extensions and other products for Glocks, SIGs, CZs, S&W M&Ps, and Beretta CX4s. (For example, their “+4” extensions for Glock 21 magazines  are very popular.) Note: The 4-round “Rawles Special” extension for the HK USP Compact .45 ACP 10 round magazine is not yet a cataloged item, so call them at: (360) 733-5649 for ordering details. Since these are special order items, you can expect a two month lead time.
Proviso: Installing one of these on a 10 round magazine in California, or in Canada and in many other hoplophobic magazine-deprived locales could land you in a heap of legal trouble .
Disclaimer (Per FTC File No. P034520 ): Taylor Freelance is not a SurvivalBlog advertiser. They have not solicited me or paid me to write any reviews or endorsements, nor have they provided me any free or reduced-price gear in exchange for this review, nor will I receive any remuneration from their sales of the product. (I paid full retail for my mag extensions.)